Asynchronous successive approximation analog/digital converters (SAR-ADCs, successive approximation register analog-digital converters) convert analog input signals into digital output signals.
For this purpose, an analog input signal is compared with the output of a digital/analog converter, in which case the output of the digital/analog converter is successively adapted in a search process on the basis of comparator decisions. The comparator and an approximation register, which provides the digital/analog converter with the values, are usually clocked by a synchronous source. The digital/analog converter must have sufficiently settled in each clock period, that is to say voltages and the like must have at least approximately settled to steady-state values, and the decision of the comparator must be made. In the case of low differential input signals at the comparator, comparator decisions may need a longer time than in the case of large differential input signals. It must therefore be ensured that a sufficiently long time is available for the comparator decisions in order to obtain correct results of the analog/digital conversion. In asynchronous SAR analog/digital converters, there is no external clock signal for the above search process. A circuit detects decisions of the comparator and generates a new clock pulse for controlling a next decision after a predefined delay time. On average, this allows the comparator to be operated at a higher speed since the clocking is based on an actually required time for the comparator decision and need not be selected, as in the case of fixed (synchronous) clocking, in such a manner that a longest possible decision time is covered.
A challenge may lie in determining a suitable delay between a comparator decision and the generation of the next comparator clock pulse. The delay must be long enough to provide the digital/analog converter with sufficient time for settling after a changeover operation. On the other hand, a low delay value allows a longer period for longer comparator decisions with the same total duration of a run of the search process.